Image transmission systems of this kind, also known as relay lens systems, are used in particular for endoscopes but also for other viewing tubes such as technoscopes, cystoscopes, periscopes, and the like. Endoscopes consist of an eyepiece part, which contains the eyepiece, as well as an elongated shaft in which additional optical components are positioned. These other optical components consist of the lens mounted on the front (distal) end of the endoscope and image transmission or relay lens systems. The lens receives the scene to be observed and produces an interim image in an initial inter-image plane. This interim image is again pictured by an image transmission or relay lens system and carried to the next inter-image plane, from where an additional image transmission or relay lens system assumes the task of forwarding to the next inter-image plane, until the image reaches the eyepiece.
The use of rod lenses to form image transmission systems was first proposed by Professor Hopkins in GB-PS 954 629. According to this theoretical proposal by Professor Hopkins, every image transmission system consists of two biconvex rod-shaped lenses and at least two additional meniscus lenses. The combination of rod-shaped biconvex lenses with meniscus lenses permits, first of all, the achievement of great image brightness, and second, a sufficiently good compensation for image errors such as astigmatisms and image field curvature that disturb image transmission systems.
On the basis of Professor Hopkins's efforts, a number of modified image transmission systems were developed and described, in which rod-shaped lens elements were also used.
An additional image transmission system of this type is familiar, for instance, from EP-B1-0 310 640. In this well-known refined image transmission system the rod lenses consist of convex/concave lenses, whose concave surfaces are turned toward the center plane and which are each cemented together with one additional lens system. The additional lens systems are biconvex single lenses, where the concave surfaces of the rod lenses are each cemented together with one of the convex surfaces of the other lens systems. Although this familiar image transmission system permits a good correction of the astigmatism and image field curvature, conventional rod lenses have the disadvantage that they generally have two spherical terminal surfaces, which are manufactured individually and are also cemented to at least one additional lens. This requires a complex, costly production process.
An additional image transmission system is known from EP 0452 053 A1. In this familiar lens system, each relay lens system consists of two identically built lens systems with one middle biconvex lens and two convex/concave lenses cemented together with this middle lens. The biconvex middle lens in this familiar lens system can be produced from a glass sphere out of which a cylindrical rod with two convex terminal surfaces is produced through appropriate processes such as for instance polishing. Because of the many spherical cementing surfaces between the middle lenses and the convex/concave lenses connected with them, this familiar lens system too can only be produced at considerable expense, since the surfaces to be joined together must be processed with great accuracy.
The problem of the many spherical cement surfaces also arises with the image transmission system known from FIG. 8 of U.S. Pat. No. 5,095,359, in which with two biconvex lenses are connected by way of a biconcave middle lens and both biconvex lenses at their ends are connected with a convex/concave lens.
On the basis of this technological status, the invention is based on the task of producing an image transmission system of the kind mentioned at the outset, which can be produced with the simplest possible rod lenses and which ensures a good correction of the image field curvature. In terms of the process, the invention is based on the task of developing an economically feasible process for producing a rod lens for an image transmission system.
The product solution of this task, in accordance with the invention, is characterized in that at least one rod lens is formed as a plano-convex lens, whose length corresponds to the curvature radius of its convex terminal surface.
This formation of the rod as plano-convex lenses in accordance with the invention permits a simple construction of the image transmission system. Rod lenses formed in this manner can be produced, for instance, at reasonable cost from one-piece biconvex lenses whose length is twice as great as the curvature radius of the two convex terminal surfaces, which are severed in order to produce two plano-convex rod lenses along the center plane.
In accordance with a practical demonstration of the invention, the plane terminal surface of at least one plano-convex rod lens forms the beginning and/or end of the respective image transmission system.
In accordance with a preferred demonstration of the invention, the rod lenses of two sequentially arranged image transmission systems are cemented together at their plane terminal surfaces. Through this direct joining of two rod lenses, it is possible to eliminate an additional glass-air surface from the image transmission system.
A considerable cost advantage and a clear simplification of the production process can be achieved, according to the invention, because two sequentially arranged rod lenses are formed as one-piece biconvex rod lenses, whose length is twice as large as the curvature radius of the two convex terminal surfaces, where one half of the biconvex rod lens is subordinated to the directly adjoining image transmission system. This type of formation of two rod lenses as one-piece biconvex lens is possibly without any substantive changes to the image transmission system because the plane terminal surfaces of the rod lenses produce no optically relevant effect and can be given up for the sake of a simply and economically produced one-piece biconvex rod lens. In comparison with conventional image transmission systems, this system is also very well adapted to the correction of image field curvature, especially in the tangential direction.
In addition, it is proposed with this invention, that the minimum of one additional lens system should be positioned symmetrically between two rod lenses with their convex terminal surfaces turned toward one another.
The minimum of one additional lens system, in accordance with the invention, consists of at least three lenses, whereby this lens system for best advantage should be constructed symmetrically and the refractive power of the two outer lenses has the opposite mathematical sign to that of the minimum of one middle lens.
In accordance with an initial demonstration of the invention, the additional lens system is formed as a threesome consisting of three lenses cemented together. Through this design of the additional lens system as a threesome consisting of three lenses cemented together in accordance with the invention, the number of glass-air surfaces of the image transmission system is further reduced.
In accordance with a second demonstration of the image transmission system of this invention, it is proposed that the additional lens system should be formed as two pairs, each consisting of two lenses cemented together. In this manner, an air space can be ensured between the two pairs. As an alternative variant to the formation of the minimum of one additional lens system as a threesome, the use of the two pairs has two additional glass-air surfaces and thus a loss of reflectivity of light energy, but this formation of the image transmission system using the rod lens, as proposed by the invention, is cost effective and simple to produce and permits a very good correction of image field curvature.
The shape of the middle lens (in the threesome) or of the middle lenses (in the pairs) depends on the type of glass chosen. In accordance with an initial practical demonstration, under the condition that nm<na the outer lenses of the additional lens system are formed as convex/concave meniscus lenses and the one (threesome) or two (pairs) middle lens(es) are formed as biconvex lenses, where nm is the refractive index of the middle lens(es) and na is the refractive index of the outer lenses.
Under the condition that nm>na 
In accordance with a second practical demonstration, the outer lenses of the additional lens system are formed as biconvex lenses and of the (threesome) or two (pairs) middle lens(es) are formed as biconcave lens(es), where nm is the refractive index of the middle lens(es) and na the refractive index of the outer lenses.
In accordance with a particularly advantageous demonstration of the additional lens system formed as a threesome, the middle lens of the additional lens system is formed as biconvex lenses, whose length is twice as great as the curvature radius of its two terminal surfaces. Just as with the formation of the rod lenses as one-piece biconvex lenses, the formation of the middle lens of the threesome as a biconvex lens is particularly cost effective.